This project deals with the problem of detection, characterization, and elucidation of the regulation of RNA tumor virus genes in host cells. Chicken leukosis, sarcoma, and endogenous proviral sequences will be detected by RNA-DNA hybridization reactions between viral genomic RNA and host cell DNA. A technique of competitive hybridization will be used to measure nucleotide sequence relationships between viruses with different biological properties and to map the position on the viral genome of sequence differences discovered. A tissue culture system for studying regulation of growth of endogenous viruses in chicken cells will be developed, and viral genetic elements required for regulation by the cell identified.